1. Field of the Invention
The present disclosure relates to a method of determining an exposure control value for a vehicle-mounted camera, which is appropriate for recognizing a target in an image in the vicinity of the vehicle taken by the camera.
2. Description of Related Art
It is known to mount a camera on a vehicle to take images in the vicinity of the vehicle. For example, the camera is mounted on the vehicle to take images ahead of the vehicle for the purpose of recognizing a white line painted on a road surface or a three-dimensional object such as a preceding or oncoming vehicle. Incidentally, it is not possible to determine with a high degree of accuracy a distance to a recognized object based on the coordinates of the object in an image taken by a vehicle-mounted camera due to installation tolerance in the optical axis of the camera. Accordingly, optical axis adjustment has to be performed for every vehicle at vehicle manufacturing factories or vehicle dealers. Generally, a target of a pattern including bright portions (white areas) and a dark portion (black area) is disposed in a predetermined position with respect to a vehicle-mounted camera, and optical-axis adjustment is performed based on the coordinates of these portions in an image taken by the camera.
However, to perform the optical-axis adjustment with a high degree of accuracy, the exposure control value for the camera has to be set to a value appropriate for recognizing the target. The exposure control value appropriate for recognizing the target is not constant, but varies depending on the environment of the target such as background noise. For example, in general exposure control in which the whole area of a taken image is set as a photometric area, when the background is dark, since the exposure is set aiming at the dark background, the white areas of a taken image of the target saturate, and accordingly expand more than in reality. On the other hand, when the background is bright, since the exposure is set aiming at the bright background, the contrast between the white and black areas in a taken image of the target is lowered. As explained above, if the exposure control value is not set appropriately, since it is not possible to accurately recognize the boundary between the white and black areas, the optical-axis adjustment cannot be performed with a high degree of accuracy.
Japanese Patent No. 4179142 describes a method capable of dealing with such a problem. In this method, an area within which a target is assumed to be located in an image taken by a camera mounted on a vehicle (“target location area” hereinafter) is set in advance based on the positional relationship between the camera and the target, and the exposure control value is calculated so that the mean value of the pixels included in the target location area becomes a predetermined value. Also, EP Patent No. 1074430 describes a method in which the exposure control is performed based on brightness histogram of a photometric area in a taken image excluding an object to be recognized.
The former conventional method has a problem in that since the target location area is set in advance based on the positional relationship between the vehicle-mounted camera and the target, and the size of the target, part of the background of the target may be included in the target location area due to placement error of the target. The latter conventional method is a method in which the exposure control is performed based on the brightness of the background, and accordingly has the problem described above.